[RASMB] measuring cell positions

[Borries Demeler]

> Regarding the comment that the interference optics appears to give the
> average position for the sample and reference sides, it seems to me the
> physics says the fringes can only exist where the light goes through both
> sides, so their disappearance should measure the position of whichever side
> is closest to the center of the channel rather than the average of the two.
> Further, if the averaging idea was correct then the radial calibration
> procedure for interference would not work, since the counterbalance is
> designed to put the edge at 5.85 and 6.15 cm on only one side.

I used the word "average" in quotes, because of course it isn't a true
average, it just looks that way if you compare the scans in the plot
image I sent. I suspect the FFT causes some sort of interpolation
that results in the plot image.

> However, in theory it should be possible to image the two sides separately
> by deliberately moving the delay setting (laser timing) so that only one
> side or the other is actually illuminated. You won't see any fringes under
> such conditions, of course, but nonetheless the superimposed image of the
> cell should tell you the position of that channel (and probably with more
> precision than the absorbance optics).

Is there a way to accomplish what John suggests here? It would be nice
if this could indeed be done, because the IF optics have about twice the
radial resolution as the UV optics. Is there a way to change the laser
timing with Beckman's software?

-Borries